1. Field of the Invention
The present invention relates to a developing apparatus using two-component developer in the electrophotographic process of a copying machine, printer, facsimile, etc., specifically, to a toner concentration control device using a toner concentration sensor which detects toner concentration magnetically.
2. Description of the Related Art
Generally, toner concentration detection in an image forming apparatus using a two-component developer is done by detecting the change in permeability of the developer. This is a method of determining the toner concentration by measuring the permeability of the developer for the permeability varies in accordance with the mixing ratio of toner and carrier. In the case of monochromatic developer a magnetic toner and a magnetic carrier are used and in the case of color developer a non-magnetic toner and a magnetic carrier are used.
For example, the output of a permeability sensor is compared with a predetermined reference value, and control is done for keeping the toner concentration constant by supplying toner so that the output of the permeability sensor is equal to the predetermined value.
However, it is difficult to stir the developer composed of toner and carrier, and stagnation of the developer in the proximity of the detecting face of the toner concentration detecting sensor or adhesion of the toner to the detecting face of the toner concentration detecting sensor occurs in a prolonged period of use, inducing erroneous detection of toner concentration. As a result, the control of the toner concentration is not performed properly resulting in varying concentration of toner in the developer to low or high value, and problems such as reduction in print density, scattering of toner, and carrier dragging are induced.
Several methods for evading the problems mentioned above are proposed in Japanese Unexamined Published Patent Application No. 63-106679 (U.S. Pat. No. 4,901,115), No. 1-261683 (U.S. Pat. No. 4,956,669), No. 2-64560 (U.S. Pat. No. 4,974,024), No. 3-256082 (U.S. Pat. No. 5,216,470), and No. 7-306592. In these methods, changes of condition of the developer residing on the detecting face of the sensor are produced by scraping down the developer accumulated on the detecting face positively and cyclically with a scraper to effect cyclic fluctuation of the output of the sensor, bottom of the fluctuation is detected, and toner concentration is determined by measuring the output in the stable range of the output after the bottom of the cyclic fluctuation; or cyclic fluctuation is produced by scraping the toner in the proximity of the detecting face of the sensor, the cyclic fluctuation is superimposed on the output when the scraping is not done to enable the control of the toner concentration over an extended period of time without stagnation of toner in the proximity of the detecting face of the sensor.
However, by these methods of detection of toner concentration, there may occur the case where the proper measurement of toner concentration is not obtained because of the distortion of the output of the toner concentration detecting sensor according as what shape the agitating mixer rib is formed and where the toner concentration detecting sensor is positioned, or the bottom of the cyclic fluctuation of the output of the sensor is not properly determined, resulting in erroneous control of toner concentration.
Further, developer is susceptible to variations of environment, particularly, the output of the toner concentration detecting sensor relatively shifts according to humidity, etc., so it is needed to make it possible to stably determine toner concentration, while excluding the influence of relative variations.
In addition, as the output of a toner concentration detecting sensor varies from one product to another depending on the accuracy of constituent parts and assembling tolerances and pulverulent body such as toner and carrier is susceptible to environment, which exert influences upon the output of the toner concentration detecting sensor, it is needed to make it possible to properly and stably determine the bottom of the fluctuating output of the toner concentration detecting sensor, while excluding these influences.
The present invention is made in the light of the problems cited above. An object of the invention is to provide a toner concentration control device which can work over an extended period of time, can stably measure the concentration of toner without being influenced by the shape of the stirring mixer of developer and location of the toner concentration detecting sensor, and accordingly can stably control the concentration of toner.
Another object of the invention is to provide a toner concentration device with high reliability which can determine the concentration of toner without being influenced by the variation of environment and variations of the accuracy of constituent parts and assembling.
A further object of the invention is to provide a toner concentration control device in which the face of the permeability sensor can be wiped without reducing the performance of stirring the toner to secure reliable measurement of toner concentration, uniform mixing of toner by stirring can be effected smoothly and in a short period of time, and a good image can be obtained.
To attain the objects cited above, a toner concentration control device according to the present invention is a device having a toner concentration detection means for detecting magnetically the information of concentration of the toner in a developer vessel storing developer composed of a carrier and toner; a toner supply means for supplying the developer based on the output of the toner concentration information; a wave-forming means for forming a wave of decreasing output of the toner concentration detection means reduced through periodically scraping off the toner on the detecting face of the toner concentration detection means with, for example, a mechanical means; and an about-minimum value detection means for detecting the minimum value or its vicinity of the descending output reduced by the wave-forming means; and so configured so that a toner concentration detection retarding means is actuated from the time the minimum value or its vicinity is detected, and the toner supply means is actuated based on the detected toner concentration information after the toner concentration detection retarding means is released of action; wherein
the toner concentration detection means comprises a waveform measuring means which measures the output of the toner concentration detection means over a predetermined period longer than the period of one cycle of the scraping cycle, and an about-minimum value judging means which determines the minimum value or its vicinity of the output value measured by the waveform measuring means; and the toner concentration detection means is actuated based on the value judged as the minimum value or its vicinity by the about-minimum judging means before the toner supply means is actuated.
By the configuration cited above, the output of the toner concentration sensor is measured over a period longer than one cycle to determine the minimum value or its vicinity before the control of toner supply is started. Succeeding detection of minimum or its vicinity is performed on the basis of the value determined as above. So, stable and reliable determination of minimum value or its vicinity is possible without being influenced by the shape of the stirring mixer of developer and the distortion of waveform owing to the location of the toner concentration sensor, and without depending on the variation of environment and variations in accuracy of constituent parts and assembling.
Further, the present invention is preferable to be configured so that a detection range determined from the minimum value or its vicinity detected by the about-minimum value detection means in the preceding cycle is provided beforehand, and the about-minimum value detection means determines the minimum value or its vicinity in the cycle concerned from the output of the toner concentration detection means in the said detection range.
By the configuration cited above, as a detection range for detecting the minimum value or its vicinity is determined on the basis of the minimum value or its vicinity of the preceding cycle, stable and reliable determination of minimum value or its vicinity is possible without being influenced by the shape of the stirring mixer of developer and the distortion of wave owing to the location of the toner concentration sensor, and without depending on the variation of environment and variations in accuracy of constituent parts and assembling.
Still further, the present invention is preferable to be configured so that a detection range determined from the minimum value or its vicinity judged by the about-minimum value detection means is provided beforehand, and the about-minimum value detection means determines the minimum value or its vicinity from the output of the toner concentration detection means in the said detection range.
By the configuration cited above, as a detection range for detecting the minimum value or its vicinity is determined on the basis of the minimum value or its vicinity measured and judged as proper through measuring the output of the toner concentration sensor over a period longer than one cycle, stable and reliable determination of minimum value or its vicinity is possible without being influenced by the shape of the stirring mixer of developer and the distortion of wave owing to the location of the toner concentration sensor, and without depending on the variation of the environment and variations in accuracy of constituent parts and assembling.
Yet further, the present invention is preferable to be configured so that the minimum value or its vicinity detected by the about-minimum value detection means and the minimum value or its vicinity detected by the about-minimum value detection means in the preceding cycle are evaluated on the basis of the predetermined value for judgement, and when the result of judgement is out of the judgement criteria, the minimum value or its vicinity detected by the about-minimum value detection means in the preceding cycle is used without replacing it by the value detected by the about-minimum value detection means in the cycle concerned.
By the configuration like this, the minimum value or its vicinity is compared with the minimum value or its vicinity in the preceding cycle, and when the absolute value of difference of the values exceeds the predetermined value for judgement, renewal of the minimum value or its vicinity is not done. By this way, malfunction due to noise is prevented and exceedingly reliable minimum value or its vicinity can be determined when determining next minimum value or its vicinity, and as a result, stable and highly reliable determination of toner concentration is possible.
Yet further, the present invention is preferable to be configured so that the about-minimum value detection means is provided with a detection prohibiting means which prohibits the detection for a proper period, a detection prohibiting period is defined from the point of the minimum value or its vicinity detected by the about-minimum value detection means, and the detection prohibiting means is activated.
By the configuration like this, as the detection of minimum value or its vicinity is not done for a determined period from the time when minimum value or its vicinity has been detected, highly reliable detection of minimum value or its vicinity is possible without influenced by the distortion of waveform caused by the shape of the developer stirring mixer rib, etc. or mounting position of the toner concentration sensor, and without malfunction due to noise, resulting in an extremely stable and highly reliable detection of toner concentration.
Yet further, the present invention is preferable to be configured so that the minimum value detection means is provided with a period judging means for detecting and judging the period of the waveform of toner concentration for a proper period, the about-minimum value detection means is activated on the basis of the value judged as minimum value or its vicinity by the about-minimum value judging means, and the toner supply means is actuated when the period of the waveform of toner concentration is judged proper by the period judging means.
By the configuration like this, as whether the minimum value or its vicinity judged by the about-minimum vale judging means is proper or not is evaluated by investigating the period before the control of toner supply is started, highly reliable detection of minimum value or its vicinity and versatile control of toner concentration containing retry process, etc. are possible.
Yet further, the present invention is preferable to be configured so that the minimum value detection means is provided with a variation judging means which detects and judges the defined variation of the output of the toner concentration detection means, and the actuation of the toner supply means is prohibited when the defined variation of the output of the toner concentration is abnormal.
By the configuration like this, abnormal detection of toner concentration caused by the damage of the sensor or absence of the developing unit is evaluated properly and the control of toner concentration with high reliability is possible.
Means cited hereinbelow specify the means for generating a waveform by periodically scraping off the developer composed of a plurality components. The waveform generating means is a mechanical mixing means so composed so that the developer composed of a plurality of components is mixed by stirring with spiral projections provided on the periphery of a rotating hollow cylindrical body. The mechanical mixing means has spiral projections differing in phase by a constant angle to each other, the height of a part of each projections, the axial position of the parts corresponding to the position of the toner concentration detection means, is different from that of the remaining part of the spiral projections to allow the one part to work as a scraper for scraping off the developer on the detecting face of the toner concentration detection means and another part to work for compressing the developer on the detecting face of the toner concentration detection means, and the toner concentration detection means detects the toner concentration when the developer is compressed to control toner concentration.
By so composing, not providing a scraper composed of sponge, etc., so that the scraping is carried out by making a part of a projection provided on the periphery of the hollow cylindrical body different in height from the remaining part of the spiral projection, and the compression of the developer on the detecting face of the toner concentration detection means is carried out by a part of another projection provided with a phase difference of a constant angle, the part which impairs the axial flow of the developer is eliminated, so scraping of the detecting face of the toner concentration detection means, reliable measurement of toner concentration, and smooth and uniform mixing by stirring in a short period of time are possible, and a toner concentration control device capable of obtaining a superior image can be provided.
It is suitable to form the projections continuing on the periphery of the hollow cylindrical body including the part functioning as scraper or preferably the part functioning as scraper is attached exchangeable on the hollow cylindrical body, and further, the gap between the tip of the scraping part of the spiral projection and detecting face of the toner concentration detection means is 1 mm or smaller, preferably 0.5 mm or smaller.
By this configuration, as the spiral projections are provided continuing on all over the hollow cylindrical body, the axial flow of developer is not impaired and stirring performance is not reduced, and with exchangeable spiral projection for scraping, it can be easily exchanged in the case of its wear or damage due to any cause. More reliable scraping is possible by setting the gap between the tip of the scraping part of the spiral projection and detecting face of the toner concentration detection means is 1 mm or smaller, preferably 0.5 mm or smaller.
That the detection of toner concentration in the toner concentration control device composed like this is preferable to be provided with a toner concentration detection retarding means which retards the detection time of the toner concentration of the developer when it is compressed from the time when the output of the toner concentration detection means is minimum due to scraping off of the developer on the detecting face of the toner concentration detection means with the scraping part of the spiral projection, is as has been described in the foregoing.
By the configuration as has been described, the control of toner concentration capable of obtaining superior image is possible owing to being able to keep the timing of toner concentration detection always constant and to being able to perform reliable measurement of toner concentration.